This invention relates to an electrical discharge machining power supply and, more particularly, to an electrical discharge machining power supply with which an excellent machined surface can be obtained by stabilizing electrical discharge and improving the cut-off of electric current.
FIG. 3 is a diagram of the arrangement of a wire-cut electrical discharge machine, in which numeral 1 denotes an electrical discharge machining power supply, 2 an upper wire guide, 3 a lower wire guide, 4 an XY table, 5 a workpiece, 6 a wire electrode, 7 an X-axis servomotor, 8 a Y-axis servomotor, 9 a dielectric treating tank, 10 a CNC, and 11 a machining command tape. A wire-cut electrical discharge machine of this type is illustrated in, for example, U.S. Pat. No. 4,467,166. The electrical discharge machining power supply 1, which produces a spark discharge across the workpiece 5 and wire electrode 6, is adapted to generate a pulse-shaped voltage impressed across the wire electrode 6 and workpiece 5. The specific construction of the electrical discharge machining power supply will now be described in detail.
FIG. 4 shows a transistor-controlled capacitor discharge circuit used in the above electrical discharge machining power supply 1. The technique used here is described in, for example, "Wire-cut Electrical Discharge Machining Techniques", p. 25, edited by Nagao Saito, published by Nikkan Kogyo Shimbun. The circuit includes transistorized switching elements inserted in the charging circuit section of the discharge circuit of a capacitor C. With this circuit, the peak Ip of a discharge current can be changed by switching transistors Tr1, Tr2 through use of a control circuit. This circuit enables the power supply side and the discharge electrodes to be cut off from each other by the switching elements, so that the power supply can be opened when a discharge current is flowing between the discharge electrodes. Accordingly, in-flow of an electric current in a state where insulation recovery has not been attained can be prevented. Further, with the circuit shown in FIG. 4, the discharge current has a high peak Ip and the discharge current pulse obtained has a narrow pulse width in comparison with a transistor discharge circuit. For these reasons, the circuit is widely employed as a power supply for wire electrical discharge machines, machines for forming small holes and special-purpose machines for carbide alloys.
Thus, with a transistor-controlled capacitor discharge circuit, electrical discharge machining is possible at a high machining speed by virtue of a discharge current having a high peak value Ip and a small pulse width. Where finishing machining is concerned, however, discharge current cut-off is poor, thus leaving a trail, and the charging current of the capacitor is caused to fluctuate by leakage current across the discharge electrodes, so that stable discharge cannot be achieved.